Various optical displacement encoders are known that use a readhead having an optical arrangement that images a scale pattern to a photodetector arrangement in the readhead. The image of the scale pattern displaces in tandem with a scale member, and the movement or position of the displaced scale pattern image is detected with a photodetector arrangement. Optical encoders may utilize a self-imaging arrangement in order to detect a displacement of a scale member comprising a scale grating. The basic principle of self-images, also known as Talbot images, is described in the paper “Fourier Images: I—The Point Source” by Cowley, J. M., and A. F. Moodie, 1957, Proc. Phys. Soc. B, 70, 486, which is incorporated herein by reference. An exemplary optical encoder utilizing self-imaging is disclosed in U.S. Pat. No. 7,608,813 (the '813 patent), which is incorporated herein by reference in its entirety. Similar encoders may utilize self-images generated according to Lau effect principles, in two or three grating encoder arrangements. One example is disclosed in the paper “Optical Encoder Based on the Lau Effect” by Crespo et al., March 2000, Opt. Eng. 39(3), 817-824. Other optical encoders may utilize moiré imaging techniques. An exemplary optical encoder utilizing moiré imaging techniques is disclosed in U.S. Pat. App. No. US20130161499A1 which is incorporated herein by reference in its entirety.
Typical encoder configurations such as those disclosed in the '813 patent rely on an illumination grating with a fixed pitch, and thus cannot utilize interchangeable illumination portion components for different pitches of illumination gratings. In various encoder configurations it is desirable to provide a compact readhead which may be manufactured in a cost efficient manner with simple and interchangeable components.